Polyurethane dispersions of high molecular weight polyurethanes have been known for some considerable time and are acquiring increasing commercial significance because they enable polyurethanes to be applied without difficulty from aqueous medium [Angew. Makromolekulare Chemie, 26 (1972), pp. 85-106; Angew. Chemie, 82 (1970), pp. 53-90; Adv. in Urethane Science and Technology, Vol. 2, 109 (1973)]. Drying is accompanied solely by the evaporation of water, with the result that film- and sheet-forming systems of this type are environmentally acceptable. Conventional polyurethane dispersions are normally dispersed macromolecules whose hydrophilic components, ionic groups or non-ionic water-dispersing polyethylene oxide segments have been incorporated during synthesis of the polymers.
Hitherto, there have been no reports of stable solutions or dispersions of low molecular weight blocked polyisocyanates which would be suitable for use in the production of polyurethanes. Accordingly, it would be of considerable commercial interest to obtain aqueous solutions or dispersions of low molecular weight blocked polyisocyanates, because aqueous solutions or dispersions of this type would lend themselves to practically every application for which solutions of blocked polyisocyanates in organic solvents have hitherto been used. In conjunction with water-soluble or water-dispersible compounds containing isocyanate-reactive hydrogen atoms, the aqueous solutions or dispersions of blocked polyisocyanates could be used in particular for the production of coatings, films and impregnations. By using solutions or dispersions of blocked polyisocyanates of the type in question as additives to aqueous polyurethane dispersions, it would be possible to control the properties of the aqueous polyurethane dispersions, for example, the degree of swelling, wet strength or resistance to chemicals of the sheet-form materials produced there from by the action of heat.
The present invention provides a route to new water-soluble or water-dispersible blocked polyisocyanates. The solutions or dispersions of the blocked polyisocyanates in water are completely stable over a period of at least 3 months.